Finger biometric sensing device including error compensation circuitry and related methods

ABSTRACT

A finger biometric sensing device may include an array of finger biometric sensing pixel electrodes and a gain stage coupled to the array of finger biometric sensing pixel electrodes. The finger biometric sensing device may also include error compensation circuitry that may include a memory capable of storing error compensation data. The error correction circuitry may also include a digital-to-analog converter (DAC) cooperating with the memory and coupled to the gain stage and capable of compensating for at least one error based upon the stored error compensation data.

RELATED APPLICATION

This application claims the benefit of a related U.S. ProvisionalApplication Ser. No. 61/642,832, filed May 4, 2012, entitled “SYSTEM FORMEASURING FINGERPRINTS AT A DISTANCE FROM THE FINGERPRINT SENSOR USINGHIGHER VOLTAGE ELECTRIC FIELDS,” the disclosure of which is incorporatedby reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of electronics, and, moreparticularly, to finger sensing devices and related methods.

BACKGROUND OF THE INVENTION

Fingerprint sensors that measure the fingerprint pattern using electricfield sensing methods have become established. U.S. Pat. Nos. 5,940,526and 5,963,679, assigned to the assignee of the present application, areexamples of this type of fingerprint sensor, and the entire contents ofwhich are incorporated herein by reference. These systems measure thefingerprint pattern by establishing an electric field between the fingerand the sensor array, and measuring the spatial fluctuations in fieldstrength at the sensor array caused by the shape of the fingerprintridge and valley pattern.

In some recent applications, the sensor may desirably capture images offingerprint patterns from fingers that are farther away from the sensorarray than is typical with today's technologies. Unfortunately, as thefinger gets farther away from the sensor array (for example when arelatively thick dielectric lies between the sensor array and thefinger) the spatial field strength variations that represent thefingerprint pattern become weaker. One way to compensate for this lossof spatial pattern strength is to increase the voltage of the signalsthat generate the field between the finger and the sensor array. Thefingerprint spatial pattern strength increases proportionately.

There may be limitations, however, on how much voltage can be placed onthe finger and on the sensor array as well. When the voltages on thefinger are too high, certain persons with very sensitive fingers mayfeel that voltage as a slight tingling. This may be undesirable in aconsumer product. On the other hand, when voltages are too high on thesensor array, the sensor readout electronics may not perform adequately,for example, they may saturate and generate unacceptable noise, and mayeven be damaged.

U.S. Pat. No. 5,940,526 describes a system where a drive voltage isimpressed on the finger (through a finger drive electrode) and thesensor array reference is connected to a device ground. The electricfield is established between the voltage on the finger and a groundedsensor reference plane in arrangements known as driven finger systems.This system works well for imaging fingers over shorter distances.However, the human body has an inherent capacitance to earth ground,hence when a voltage is impressed on the finger, current flows throughthe finger and body to that ground. When the voltage on the finger isincreased, people with sensitive fingers may feel that ground current asa tingling sensation.

U.S. Pat. No. 5,963,679 describes a sensor where a drive voltage isimpressed on a reference electrode positioned beneath the sensing arrayelements, while the finger is connected to the system ground (through afinger drive electrode), in arrangements known as driven sensor systems.This system may work well for imaging fingers over short distances; andfor those systems, it maintains the finger voltage close to ground.

However two circuit related problems may currently limit thisimplementation. First, the small spatial voltage differencesrepresenting the ridge-valley pattern are now riding on top of arelatively large common mode voltage from the nearby referenceelectrode, making measurement of the small spatial voltage differencesdifficult. Second, the sensor readout electronics, fabricated withstandard economical CMOS devices, may not work properly if the voltageson the sensor array exceed the operating range of those devices.

In other words, the detected signals generated from the sensor array andbased upon placement of the user's finger adjacent the sensor array arerelatively small compared to the drive signal. Thus, these relativelysmall detected signals may be increasingly difficult to process alongwith the relatively high drive signal, limiting measurement resolutionof the detected signals, for example. Amplifier and processing stagesthat read and process the detected signals may add additional noise.Another source of noise may be fixed pattern noise from the sensorarray, which also may make it increasingly difficult measure thedetected signals.

SUMMARY

In view of the foregoing background, it is therefore an object of thepresent invention to provide a finger biometric sensing device thatcompensates for the various noise sources.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a finger biometric sensing device thatmay include an array of finger biometric sensing pixel electrodes and atleast one gain stage coupled to the array of finger biometric sensingpixel electrodes. The finger biometric sensing device also may includeerror compensation circuitry. The error compensation circuitry mayinclude a memory capable of storing error compensation data, and adigital-to-analog converter (DAC) cooperating with the memory andcoupled to the at least one gain stage. The DAC may be capable ofcompensating for at least one error based upon the stored errorcompensation data. Accordingly, the finger biometric sensing device maycompensate for various noise sources, for example, by compensating forerrors from an uneven finger contact surface, and gain and DAC settings,which may reduce noise and increase measurement sensitivity.

The memory may be capable of storing error compensation data to accountfor tilt errors, for example. The finger biometric sensing device mayfurther include a dielectric layer over the array of finger biometricsensing pixel electrodes. The dielectric layer may have a non-uniformthickness. The memory may be capable of storing error compensation datato account for the non-uniform thickness of the dielectric layer, forexample.

The memory may be capable of storing error compensation data to accountfor fixed pattern noise errors. The memory may be capable of storingerror compensation data to account for a gain stage offset error.

The at least one gain stage may include a plurality thereof coupledtogether in series and defining a summing node between a pair ofadjacent ones of the plurality of gain stages. The DAC may be coupled tothe summing node, for example.

The finger biometric sensing device may further include an outputanalog-to-digital (ADC) coupled to the at least one gain stage andhaving a dynamic range. The finger biometric sensing device may furtherinclude control circuitry capable of adjusting the at least one gainstage so that an output thereof is within the dynamic range of theoutput ADC, for example.

The finger biometric sensing device may further include switchingcircuitry coupled to the array of finger biometric sensing pixelelectrodes and the at least gain stage. The switching circuitry may becapable of sequentially generating output data for adjacent regions ofthe array of finger biometric sensing pixel electrodes, for example.

The finger biometric sensing device may further include a fingercoupling electrode adjacent the array of finger biometric sensing pixelelectrodes. A voltage generator may be coupled to the finger couplingelectrode, for example.

A method aspect is directed to a method of error compensation in afinger biometric sensing device that includes an array of fingerbiometric sensing pixel electrodes, at least one gain stage coupled tothe array of finger biometric sensing pixel electrodes. The method mayinclude using error compensation circuitry that may include a memorycapable of storing error compensation data and a digital-to-analogconverter (DAC) cooperating with the memory and coupled to the at leastone gain stage to compensate for at least one error based upon thestored error compensation data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an electronic device according to the presentinvention.

FIG. 2 is a schematic block diagram of the electronic device of FIG. 1.

FIG. 3 is a more detailed schematic block diagram of a portion of theelectronic device of FIG. 1.

FIG. 4 is schematic circuit diagram of a portion of the electronicdevice of FIG. 1.

FIG. 5 is a schematic circuit diagram of a portion of drive signalnulling circuitry in accordance with another embodiment of the presentinvention.

FIG. 6 is a schematic circuit diagram illustrating the finger couplingelectrode coupled to the device ground in accordance with an embodimentof the present invention.

FIG. 7 is a flow chart of a detailed method of error compensation inaccordance with an embodiment of the present invention.

FIG. 8 is another flow chart of a detailed method of error compensationin accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to FIGS. 1-4, an electronic device 20 is nowdescribed. The electronic device 20 illustratively includes a portablehousing 21 and a processor 22 carried by the portable housing. Theelectronic device 20 is illustratively a mobile wireless communicationsdevice, for example, a cellular telephone. The electronic device 20 maybe another type of electronic device, for example, a tablet computer,laptop computer, etc. In some embodiments, the electronic device 20 maybe an integrated circuit for use with another or host electronic device.

Wireless communications circuitry 25 (e.g. a wireless transceiver,cellular, WLAN Bluetooth, etc.) is also carried within the housing 21and coupled to the processor 22. The wireless transceiver 25 cooperateswith the processor 22 to perform at least one wireless communicationsfunction, for example, for voice and/or data. In some embodiments, theelectronic device 20 may not include a wireless transceiver 25.

A display 23 is also carried by the portable housing 21 and is coupledto the processor 22. The display 23 may be a liquid crystal display(LCD), for example, or may be another type of display, as will beappreciated by those skilled in the art. A device memory 26 is alsocoupled to the processor 22.

A finger-operated user input device, illustratively in the form of apushbutton switch 24, is also carried by the portable housing 21 and iscoupled to the processor 22. The pushbutton switch 24 cooperates withthe processor 22 to perform a device function in response to thepushbutton switch. For example, a device function may include a poweringon or off of the electronic device 20, initiating communication via thewireless communications circuitry 25, and/or performing a menu function.

More particularly, with respect to a menu function, the processor 22 maychange the display 23 to show a menu of available applications basedupon pressing of the pushbutton switch 24. In other words, thepushbutton switch 24 may be a home switch or button, or key. Of course,other device functions may be performed based upon the pushbutton switch24. In some embodiments, the finger-operated user input device 24 may bea different type of finger-operated user input device, for example,forming part of a touch screen display. Other or additionalfinger-operated user input devices may be carried by the portablehousing 21.

The electronic device 20 includes a finger biometric sensing device 50,which may be in the form of one or more integrated circuits (ICs). Thefinger biometric sensing device 50 includes an array of finger biometricsensing pixel electrodes 31 that is carried by the pushbutton switch 24to sense a user's finger 40 or an object placed adjacent the array offinger biometric sensing pixel electrodes. The array of finger biometricsensing pixel electrodes 31 is carried by the pushbutton switch 24 sothat when a user or object contacts and/or presses downward on thepushbutton switch, data from the user's finger 40 is acquired, forexample, for finger matching and/or spoof detection, as will describedin further detail below. In other words, the array of finger biometricsensing pixel electrodes 31 may cooperate with circuitry, as will beexplained in further detail below, to be responsive to static contact orplacement of the user's finger 40 or object. Of course, in otherembodiments, for example, where the array of finger biometric sensingpixel electrodes 31 is not carried by a pushbutton switch, the array offinger biometric sensing pixel electrodes may cooperate with circuitryto be responsive to sliding contact (i.e. a slide sensor), or responsiveto static placement (i.e. a standalone static placement sensor).

The array of finger biometric sensing pixel electrodes 31 isillustratively an 88×88 array of finger sensing pixel electrodes. Inother words, conceptually, the array of can be organized into 88 rows of88 pixels and defines a rectangle. Of course, in some embodiments, thearray of finger biometric sensing pixel electrodes 31 may be a differentsize. Each finger sensing pixel electrode 35 may define an electricfield sensing pixel such as disclosed in U.S. Pat. No. 5,940,526 toSetlak et al., assigned to the present assignee, and the entire contentsof which are herein incorporated by reference. Of course, in someembodiments, each finger biometric sensing pixel electrode 35 may defineanother type of finger biometric sensing pixel.

The processor 22 may also cooperate with the array of finger biometricpixel sensing electrodes 31 to determine a finger match based uponfinger biometric data. More particularly, the processor 22 may determinea finger match based upon enrollment data stored it the device memory26. The processor 22 may also determine a live finger based upon spoofdata. More particularly, the processor 22 may determine a live fingerbased upon a complex impedance and/or bulk impedance measurement.

In some embodiments, the processor 22 may cooperate with the array offinger biometric pixel sensing electrodes 31 to perform a navigationfunction, for example. Of course the processor 22 may cooperate with thearray of finger biometric sensing electrodes 31 and/or other circuitryto perform other or additional functions, as will be appreciated bythose skilled in the art.

The finger biometric sensing device 50 also includes switching circuitry32 coupled to the array of finger sensing pixel electrodes 31 and thegain stages 60 a-60 d. The switching circuitry 32 is capable ofacquiring finger biometric data from each of a plurality of sub-arrays33 a-33 f of the array of finger biometric sensing pixel electrodes 31.More particularly, the switching circuitry 32 is capable of sequentiallygenerating output data for adjacent regions of the array of fingerbiometric sensing pixel electrodes 31 or sub-arrays 33 a-33 f. In an88×88 finger biometric pixel sensing electrode array, there are 7744finger biometric sensing pixel electrodes 35 and 7744 correspondingswitches. Of course, additional switches may be used, as will beappreciated by those skilled in the art.

The finger biometric sensing device 50 also includes drive circuitry 44capable of generating a drive signal coupled to the array of fingersensing pixel electrodes 31. The array of finger sensing pixelelectrodes 31 cooperates with the drive circuitry 44 to generate adetected signal based upon placement of a finger 40 adjacent the arrayof finger sensing pixel electrodes. The gain stages 60 a-60 d arecoupled together in series and define summing nodes 61 a-61 b between apair of adjacent ones of the gain stages. A summing node 61 c is coupledto the end of the fourth gain stage 60 d.

The first gain stage 60 a may be in the form of one or more variablegain amplifiers 63 a defining front end amplifiers, each respectivelycoupled to a finger sensing electrode 35 from the array of fingersensing electrodes 31. The first gain stage 60 a is input with thedetected signal at a raw signal level. An output of the first gain stage60 a is coupled to the first summing node 61 a. For an 8-channelimplementation (e.g., for an 88×88 array of finger sensing pixelelectrodes divided into eleven 8×8 regions), there are 8 instances ofthe illustrated first gain stage 60 a.

The second gain stage 60 b may also be in the form of one or morevariable gain amplifiers 63 b defining AC amplifiers. Each amplifier 63b of the second gain stage 60 b has an input coupled to the firstsumming node 61 a. A capacitor 64 or other impedance device may becoupled between the first summing node 61 a and the second gain stage 60b. The second gain stage 60 b also processes the input signal at a rawsignal level. For an 8-channel implementation (e.g., for an 88×88 arrayof finger sensing pixel electrodes divided into eleven 8×8 regions),there are 8 instances of the illustrated second gain stage 60 b.

The third gain stage 60 c may be in the form of one or more pairs ofvariable gain amplifiers 63 c, 63 d defining a correlated double sampler(CDS). More particularly, the third gain stage 60 c may include odd andeven variable gain amplifiers 63 c, 63 d for each channel. For an8-channel implementation, there are 8 instances of the illustrated thirdgain stage 60 c. The output of each of the odd and even variable gainamplifiers 63 c, 63 d of the third gain stage 60 c are input to amultiplexer 66. As will be appreciated by those skilled in the art, themultiplexer 66 may be a 16:1 multiplexer for an 8 channelimplementation. The output of the multiplexer 66 is summed, at thesecond summing node 61 b, with an output from a second digital-to-analogconverter (DAC) 72, which will be described in further detail below. Thethird gain stage 60 c also processes the input signal at a raw signallevel.

The fourth gain stage 60 d may also be in the form of one or morevariable gain amplifiers 63 e. The variable gain amplifier 63 e may havean input coupled to the second summing node 61 b and an output coupledto the third summing node 61 c. The fourth gain stage 60 d processes theinput signal at a feature signal level. Of course, while four gainstages 60 a-60 d are illustrated and described, there may be additionalgain stages.

The drive circuitry 44 includes a finger coupling electrode 47 adjacentthe array of finger sensing pixel electrodes 31. The drive circuitry 44also includes a drive signal generator, which in the illustratedembodiment is in the form of a voltage generator 48, coupled to thefinger coupling electrode 47. The array of finger sensing pixelelectrodes 31 and the gain stages 60 a-60 d have a circuit reference 52associated therewith. The circuit reference 52 is to be coupled to adevice ground 53 so that the voltage generator 48 drives the fingercoupling electrode 47 with respect to the circuit reference 52 and thedevice ground 53.

The finger biometric sensing device 50 also includes drive signalnulling circuitry 45 coupled to the gain stages 60 a-60 d. As will beappreciated by those skilled in the art, relatively high voltage drivesignals may result in relatively large common mode voltages appearing onthe detected signal generated from the array of finger biometric sensingpixel electrodes 31. Since the drive signal generally carries no usefulinformation, it may be particularly desirable to reduce or eliminate itas early as possible in the signal chain. Specifically, small spatialvariations in electric field intensity in the presence of a relativelylarge average field intensity may be measured.

The drive signal nulling circuitry 45 is capable of reducing therelatively large drive signal component from the detected signal. Thedrive signal nulling circuitry 45 includes digital-to-analog converter(DAC) 46 capable of generating an inverted scaled replica of the drivesignal for the gain stages 60 a-60 d. More particularly, the DAC 46 iscoupled to the first summing node 61 a. A memory may be coupled to theDAC 46.

In some embodiments, for example, as illustrated in FIG. 5, the drivesignal nulling circuitry 45′ includes an inverting amplifier 67′ andimpedances Z1, Z3 coupled thereto. In such a drive signal nulling system45′, the sense amplifier 63 a′ (i.e., first gain stage 60 a′), which maybe a differential amplifier, is configured as an operational amplifiersummer. The summing node 68′ is connected to the finger sensing pixelelectrode 35′ via the impedance Z1, which may be relatively large (e.g.,a small capacitor) and to an inverted version of the pixel drive signal(the drive signal cancellation signal), also via a summing impedance. Aswill be appreciated by those skilled in the art, the summing impedancesZ1, Z3 may be considered drive signal cancellation summing impedances.Other impedances also exist, for example, a finger coupling electrodeimpedance Ze coupled between the user's finger 40′ and the fingercoupling electrode 47′, which in turn is coupled to a device ground 53′.A finger impedance Zf exists between the user's finger 40′ and thefinger biometric sensing pixel electrode 35′, a reference impedance Zgbetween finger sensing biometric sensing pixel electrode and thesubstrate 49′ of a first IC. A reference impedance Zr is coupled betweenthe circuit ground 52′ of a second IC and the non-inverting input of theamplifier 63 a′ also on the second IC. An impedance Z2 may also becoupled across the output and the inverting input of the amplifier 63a′. A voltage regulator 66′ may also be coupled between the voltagesupply Vcc and the circuit ground 52′ which is coupled to the deviceground 53′ in the illustrated embodiment.

The large drive signal on the sensor plate of each finger sensingbiometric sensing pixel electrode 35′ and the drive cancellation signalbalance at the current summing node 68′ of the amplifier 63 a′ so thatthe signals at the amplifier are relatively low voltage signals with therelatively high voltage drive signal removed. In this embodiment, thecomponents that may have high voltage capability are those that generatehigh voltage drive signals. None of the measurement circuitry may havehigh voltage capabilities. A potential disadvantage of this approach mayinclude tracking of the drive signal cancellation signal to the sensordrive signal so that approximate balance is maintained at the sense ampsumming nodes 68′ under an increased amount of conditions.

Referring again to FIGS. 1-4, the drive circuitry 44 may be capable ofgenerating a square wave at an amplitude in a range of 10 to 20 voltsand at a frequency in a range of 1 to 5 MHz, for example. Of course, thedrive circuitry 44 may be capable of generating a different type of wavehaving a different amplitude range and in a different frequency range,as will be appreciated by those skilled in the art. As will beappreciated by those skilled in the art, a relatively high voltage drivesignal may be managed by floating the ground of the amplifier in thefirst gain stage 60 a, i.e., sense amplifier 63 a, and connecting it tothe drive signal.

Referring now briefly to FIG. 6, in another embodiment, the fingercoupling electrode 47″ is coupled to a device ground 53″ so that thevoltage generator 48″ drives the circuit reference 52″ with respect tothe finger coupling electrode and the device ground. A finger couplingelectrode impedance Ze is defined between the user's finger 40″ and thefinger coupling electrode 47″, which in turn, is coupled to the deviceground 53′. A finger impedance Zf exists between the user's finger 40″and the finger biometric sensing pixel electrode 35″. A referenceimpedance Zg exists between the finger biometric sensing pixel electrodeand the substrate 49″ of a first IC. A reference impedance Zr existsbetween the circuit ground 52″ of a second IC and the non-invertinginput of the amplifier 63 a″ also on the second IC. A voltage regulator66″ may also be coupled to between the voltage supply Vcc and thecircuit ground 52″ or device ground 53″.

In contrast to driving the finger coupling electrode 47″, the relativelyhigh voltage drive signal may not be seen by the first gain stage 60 a″or sense amplifier 63 a″ between its inputs and its ground. Smalldifference voltages may be visible to the first gain stage 60 a″ orsense amplifier 63 a″. However, an output signal from the senseamplifier 63 a″ may not be ground referenced, but rather is riding ontop of the internal excitation signal or drive signal. Active levelshifting circuitry may be applied in the subsequent signal path toremove the drive signal and generate a ground referenced output.

As will be appreciated by those skilled in the art, one benefit of thisapproach is that the drive signal may be easier to remove after a smallridge pattern signal has been amplified by the sense amplifier 63 a″.For example, the drive signal cancellation techniques discussed abovemay be easier to apply downstream of the sense amplifier 63 a″. Thistype of floating ground system may be implemented in a variety of waysdiscussed below.

One way of implementing a floating ground system as noted above is via amulti-chip or integrated circuit (IC) approach, for example, asillustrated in FIGS. 5 and 6. In this implementation high voltages arehandled by one silicon die and the lower voltages by a second die. Forsome of the systems described above, for example, the substrates of thedifferent die may also operate at different voltages. The system may bepartitioned in a variety of different ways based upon relative costs ofthe different die. For example, in one embodiment, the drive signalgeneration and the array of finger biometric sensing pixel electrodecircuitry are fabricated in a high voltage process, while the downstreamsignal processing and analog to digital conversion circuitry isfabricated in a low voltage process. Alternatively, in anotherembodiment, the array circuitry and signal processing circuitry may befabricated in a low voltage process and run with a driven ground, whilethe drive signal generation and the data conversion and interfacingcircuitry is fabricated in a high voltage process.

Another way of implementing a floating ground system is via a singlechip. In this implementation, a high voltage IC fabrication process withlow voltage device options is utilized for lower cost and increasedefficiency performance. In particular, both the drive signal generationcircuitry and sensing front end circuitry are implemented with highvoltage technology components. An example of this system includes a Psubstrate tied to circuit ground and an N+ buried layer and deep n wellside wall junctions to isolate all active p well regions. A DC-to-DCboost converter is implemented that takes typically a 1.8/3.3V supplyinput and generates a 15-20V output. This output voltage is connected toall the N+ buried layers and n well side wall junctions for reverse biasrelative to all active p well regions. An appropriate drive circuit(e.g. buffer) couples to this boost output voltage and drives the p wellregions of the frontend sensing circuits. This may include pixel andmultiple frontend amplifier stages. The signals from these frontendamplifiers are then appropriately conditioned (e.g. ac coupled, levelshifted, etc., for example, using the techniques described above) beforethey are connected to lower voltage signal processing circuitry. The pwell regions of these lower voltage circuits are connected to circuitground. In the same way, digital circuits involved in digital signalprocessing, control, memory, etc. have their p well regions connected tocircuit ground. Recall that all circuit regions are isolated by the highvoltage N+ buried layer and n well side wall junctions which are tied tothe highest potential from the boost converter.

The advantages of this implementation with respect to systems using twoseparate silicon die may be that a single part has signal generation andsensing included. Moreover, only the frontend sensing circuitry isdriven by the boosted high voltage signal instead of the entire die,which may be particularly advantageous since a smaller driven area maylower power and reduce potential interference issues, andsynchronization of finger drive and control signals may be simplified asall are on-chip with lower parasitics. Additionally, an external digitalinterface may not be implemented in a special fashion thus allowing forcontinuous die control from the host processor which may be particularadvantageous for navigation applications. Still further, overall systemnoise may be lowered as power is lower and interfacing is simplified.

The finger biometric sensing device 50 may also include a dielectriclayer 55 covering the array of finger sensing pixel electrodes 31 (FIG.2). An output analog-to-digital converter (ADC) 56 may be coupleddownstream from the gain stages 60 a-60 d. More particularly, the outputADC 56 may be coupled to the fourth gain stage 60 d and may have adynamic range. In some embodiments, a memory may be coupled to the ADC56. Control circuitry 57 is capable of adjusting the fourth gain stage60 d, and in some embodiments, other and/or additional gain stages sothat an output thereof is within the dynamic range of output ADC 57.

The finger biometric sensing device 50 may further include errorcompensation circuitry 70. The error compensation circuitry 70 mayinclude a memory 71 capable of storing error compensation data and thesecond digital-to-analog converter (DAC) 72 cooperating with the memoryand coupled to the second summing node 61 b between the third and fourthgain stages 60 c, 60 d. The second DAC 72 may be capable of compensatingfor at least one error based upon the stored error compensation data. Inparticular, the memory 71 may be capable of storing error compensationdata to account for tilt errors. Alternatively or additionally, wherethe dielectric layer 55 has a non-uniform thickness, the memory 71 maybe capable of storing error compensation data to account for non-uniformthickness errors. For example, the dielectric layer 55 may have a largerthickness in one location than in another location and may vary between100 μm-500 μm. This may be a result of the manufacturing process, forexample, or may be intentional, for example, where a curved shapedielectric layer 55 is desired. For example, for a concave shapeddielectric layer 55, the change in thickness of the dielectric materialabove the array of finger sensing pixel electrodes 31 or sensor die maycause the detected signal to vary in both offset and amplitude.

This variation in offset and amplitude may be compensated in the digitaldomain by applying a digital offset and gain to the detected signal orsensor data. The compensation values may be stored in a table (i.e.,look-up table) in the memory 71. In an extreme case, for example, eachfinger biometric pixel electrode 35 may have a unique offset and gain.Correction values from the memory 71 would be applied to a number ofpixels in a local region (i.e. applied for some number of rows andcolumns).

Another approach to compensating for offset and amplitude variations maybe to measure local minimum and maximum values and apply linearinterpolation between the measured features. For example, even with aflat dielectric layer 55 or surface, tilt or pressure gradients cancause a gradient to be introduced in the measured levels of the detectedsignal. First and second voltages are used to calculate the localdigital gain and offset (G1, O1) that are to be applied to increase thesignal in the dynamic range of the output data. Third and fourthvoltages are used to calculate the desired local digital gain and offset(G2, O2). Using a linear interpolation between the measured values (suchthat the values start with G1, O1 and linearly increase or decrease toG2, O2) removes the gradient in the signal level. A variation of thismay also be used to compensate for measurement channel differences, forexample, at the same time.

In one example, consider a four channel system where channel 1 is usedto acquire data from column 1 in each row, channel 2 is for column 2,channel 3 is for column 3, and channel 4 is for column 4. Applying alinear interpolation independently along each column, for example,reduces or may remove gradients and channel-to-channel gain and offsetdifferences without an additional processing step. Of course, in someembodiments, channel-to-channel noise may be removed by using a singlechannel.

The memory 71 may also be capable of storing error compensation data toaccount for fixed pattern noise errors. As will be appreciated by thoseskilled in the art, as imaging distances increase, for example, betweena user's finger 40 and the array of finger biometric sensing pixelelectrodes, (by way of the dielectric layer 55, for example), fixedpattern noise also increasingly becomes problematic. To account forthis, compensation data accounting for the fixed pattern noise may beloaded into the memory 71 during production.

The memory 71 may further be capable of storing error compensation datato account for a gain stage offset error. As will be appreciated bythose skilled in the art, when using series coupled gain stages, DCoffset voltages may be added at each gain stage. Thus, it may betypically desirable to address these DC offset voltages across the gainstages.

The memory 71 may further be capable of storing error compensation datato account for pixel-to-pixel variation. More particularly,pixel-to-pixels differences or deltas may be particularly visible in anoise floor at a relatively high gain. Offsets between the pixels may bemeasured at the high gain setting. The difference or delta between thesmallest offset with respect to the rest of the array is measured. Thisresult may be scaled based upon the current gain settings of the gainstages 60 a-60 d and stored in the memory 71 as compensation data. Itwill be appreciated that in some embodiments, an additional memory maybe coupled to other and/or additional components and/or store other oradditional compensation data.

As will be appreciated by those skilled in the art, error compensation,for example, gain control, may be particularly advantageous to increasecontrast and reduce saturation. In some embodiments, error compensation,for example, gain control, may operate either globally or regionally.With respect to global gain control samples are taken from the center ofthe array of finger sensing pixel electrodes and used to determine thesetting of the gain stages 60 a-60 d and the DAC 72. The same gain andDAC settings may be used to acquire each region. With respect toregional gain control, samples are taken from the region being acquiredto determine the settings of each gain stage 60 a-60 d and the DAC 72.The gain and DAC settings are applied for that region and are typicallydifferent from region to region.

In some embodiments, initial gain settings may be based upon previousgain settings. For regionally based gain compensation, the initialsettings for a given region in a given sub-array may be based upon aprevious region, with the exception of a first region, which may bebased on the first region in the previous sub-array, for example. Forglobally based gain compensation, the initial setting may be based uponthe previous frame settings. However, in either case, the initialsettings may be the based upon the previous values of the DAC 72 and theprevious gain index minus one.

Additionally, as will be appreciated by those skilled in the art, inaddition to the circuitry described above, power control circuitry 77may also be included for control and management of the power. The powercontrol circuitry 77 may be particularly advantageous in portable orbattery powered electronic devices, for example.

Finger detection circuitry 78 may also be included and coupled to thepower control circuitry 77. The finger detection circuitry 78 may detectthe presence of a user's finger 40 adjacent the array of fingerbiometric sensing pixel electrodes 31. The finger detection circuitry 78may cooperate with the power control circuitry 77 to power othercircuitry, for example, as described above, based upon detection of auser's finger. As will be appreciated by those skilled in the art, thefinger detection circuitry 78 may be based upon a frequency based fingerdetection technique and/or an image based finger detection technique. Ofcourse, other or additional techniques may be used.

A method aspect is directed to a method of reducing a relatively largedrive signal in a finger biometric sensing device 50 that includes drivecircuitry 44 capable of generating a drive signal an array of fingerbiometric sensing pixel electrodes 31 cooperating with the drivecircuitry and capable of generating a detected signal based uponplacement of a finger adjacent the array of finger biometric sensingpixel electrodes. The detected signal includes a relatively large drivesignal component and a relatively small sense signal componentsuperimposed thereon. The finger biometric sensing device 50 includesgain stages 60 a-60 d coupled to the array of finger sensing biometricpixel electrodes 31. The method includes operating drive signal nullingcircuitry 45 coupled to the gain stages 60 a-60 d to reduce therelatively large drive signal component from the detected signal.

Another method aspect is directed to a method of error compensation in afinger biometric sensing device 50 that includes an array of fingerbiometric sensing pixel electrodes 31 and gain stages 60 a-60 d coupledto the array of finger biometric sensing pixel electrodes. The methodincludes using error compensation circuitry 70 that includes a memory 71capable of storing error compensation data and a digital-to-analogconverter (DAC) 72 cooperating with the memory and coupled to the atleast one gain stage to compensate for at least one error based upon thestored error compensation data.

Yet another method aspect is directed to a method of making anelectronic device 20 that includes circuitry carried by a housing 21that has a device ground 53 associated therewith. An array of fingerbiometric sensing pixel electrodes 31 is also carried by the housing 21and has a circuit reference 52 associated therewith. The method includescoupling a finger coupling electrode 47 adjacent the array of fingerbiometric sensing pixel electrodes 31 to the device ground 53 andcoupling a drive signal or voltage generator 48 between the deviceground 53 and the circuit reference 52.

Referring now to the flowchart 100 in FIG. 7, a more detailed errorcompensation method is now described. Beginning at Block 102, the methodincludes adjusting the setting of the DAC 72, the second gain stage 60b, and the third gain stage 60 c. More particularly, at Block 104, themethod includes acquiring black, white, and average pixel values. AtBlock 106, the method includes finding the midpoint of the black andwhite pixels, or the largest and smallest pixel values. In someembodiments, the fourth largest and smallest pixel values may be used toallow for some tolerance for bad pixels, as will be appreciated by thoseskilled in the art. A delta ADC value is calculated at Block 108 and maybe based upon a midpoint of the value of the largest and smallest pixelvalue. The method includes calculating the delta DAC value based uponthe delta ADC value and a DAC factor (Block 109). If, as determined atBlock 110, the midpoint value is greater than a threshold value, forexample 0x80, the delta DAC value is subtracted from a DAC value (Block112). Otherwise, the delta DAC value is added to the DAC value (Block114). The subtraction result is clamped to 0, and the addition result isclamped to 0x3FF. Of course, the results may be clamped to other values.

At Block 116, the results are checked for saturation, i.e., whether theblack and white pixel values are 0 or 0xFF, for example. If the resultsare saturated and the gain index is not equal to 0, then the gain indexis decremented (Block 118) and the method returns to Block 104.

At Block 120, the method includes calculating a feature value bysubtracting the white pixel value from the black pixel value. A gainincrease is calculated based upon a feature target value being dividedby the feature value (Block 122). The memory 71, and more particularly,a look-up table in the memory is accessed with gain factors to determinethe gain index increase. The method ends at Block 124.

Referring now to the flowchart 130 in FIG. 8, beginning at Block 132 asecond series of steps adjust the DAC offset, the gain of the fourthgain stage 60 d and the dynamic range of the ADC 56 (high voltagereference). At Block 134, the method includes acquiring black, white,and average pixel values. At Block 136, the method includes finding themidpoint of the black and white pixels, or the largest and smallestpixel values. In some embodiments, the fourth largest and smallest pixelvalues may be used to allow for some tolerance for bad pixels, as willbe appreciated by those skilled in the art.

The method includes calculating the DAC offset based upon the delta ADCand a DAC offset factor (Block 138). The delta ADC may be based upon amidpoint of the value of the largest and smallest pixel value. If themidpoint value is greater than a threshold value, for example 0x80,(Block 140) the delta DAC offset value is added to the DAC offset value(Block 142). Otherwise, the delta DAC offset value is subtracted fromthe DAC offset value (Block 144). The subtraction result is clamped to0, and the addition result is clamped to 0x3FF. Of course, the resultsmay be clamped to other values.

At Block 146, the method includes calculating a feature value bysubtracting the white pixel value from the black pixel value. A gainincrease value is calculated based upon a second feature target valuebeing divided by the feature value (Block 148). The memory 71, and moreparticularly, a look-up table in the memory is accessed with gainfactors to determine the gain index increase of the fourth gain stage 60d (Block 150).

At Block 152, the feature values are calculated after a gain increase.The dynamic range of the ADC 56, and more particularly the high voltagereference, for obtaining a post gain feature value of the second featuretarget value is calculated (Block 154). The high reference setting toprovide the high voltage reference of the ADC 56 is looked up in thememory 71 (Block 156). The method ends at Block 158.

An additional sample of pixel value data may be obtained to verify finalsettings, for example. This may also be used to make fine adjustments tothe DAC offset and ADC dynamic range. If, for example, the finalsettings show saturation, it may be desirable to restart gain controlprocess rather than attempt one or more fine adjustments. Restarting thegain control process may allow settings to be determined in threesamples, while making fine adjustments to reduce saturation generallytakes an indeterminate number of samples since the degree of saturationis unknown.

Additionally, while the device has been described with respect to twomemories 26, 71, it will be appreciated by those skilled in the art thata single shared memory may be used, or other or additional memorydevices may be used. Moreover, while different compensation techniqueshave been described herein, for example, with respect to drive signalcompensation, tilt compensation, and gain compensation, it will beappreciated by those skilled in the art that any of the techniques maybe used alone or in combination of any or all of the other compensationtechniques. Other compensation techniques may also be used inconjunction with those described herein. The compensation techniques maybe performed in parallel or serially, and may be processed by more thanone processor, for example.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A finger biometric sensing devicecomprising: an array of finger biometric sensing pixel electrodes; atleast one gain stage coupled to said array of finger biometric sensingpixel electrodes; and error compensation circuitry comprising a memorycapable of storing error compensation data, and a digital-to-analogconverter (DAC) cooperating with said memory and coupled to said atleast one gain stage and capable of compensating for at least one errorbased upon the stored error compensation data.
 2. The finger biometricsensing device according to claim 1 wherein said memory is capable ofstoring error compensation data to account for tilt errors.
 3. Thefinger biometric sensing device according to claim 1 further comprisinga dielectric layer over said array of finger biometric sensing pixelelectrodes; wherein said dielectric layer has a non-uniform thickness;and wherein said memory is capable of storing error compensation data toaccount for the non-uniform thickness of said dielectric layer.
 4. Thefinger biometric sensing device according to claim 1 wherein said memoryis capable of storing error compensation data to account for fixedpattern noise errors.
 5. The finger biometric sensing device accordingto claim 1 wherein said memory is capable of storing error compensationdata to account for a gain stage offset error.
 6. The finger biometricsensing device according to claim 1 wherein said at least one gain stagecomprises a plurality thereof coupled together in series and defining asumming node between a pair of adjacent ones of said plurality of gainstages; and wherein said DAC is coupled to said summing node.
 7. Thefinger biometric sensing device according to claim 1 further comprising:an output analog-to-digital (ADC) coupled to said at least one gainstage and having a dynamic range; and control circuitry capable ofadjusting said at least one gain stage so that an output thereof iswithin the dynamic range of said output ADC.
 8. The finger biometricsensing device according to claim 1 further comprising switchingcircuitry coupled to said array of finger biometric sensing pixelelectrodes and said at least one gain stage capable of sequentiallygenerating output data for adjacent regions of said array of fingerbiometric sensing pixel electrodes.
 9. The finger biometric sensingdevice according to claim 1 further comprising a finger couplingelectrode adjacent said array of finger biometric sensing pixelelectrodes, and a voltage generator coupled to said finger couplingelectrode.
 10. A finger biometric sensing device comprising: an array offinger biometric sensing pixel electrodes; a finger coupling electrodeadjacent said array of finger biometric sensing pixel electrodes; avoltage generator coupled to said finger coupling electrode; at leastone gain stage coupled to said array of finger biometric sensing pixelelectrodes; error compensation circuitry comprising a memory capable ofstoring error compensation data, and a digital-to-analog converter (DAC)cooperating with said memory and coupled to said at least one gain stageand capable of compensating for at least one error based upon the storederror compensation data; an output analog-to-digital (ADC) coupled tosaid at least one gain stage and having a dynamic range; and controlcircuitry capable of adjusting said at least one gain stage so that anoutput thereof is within the dynamic range of said output ADC.
 11. Thefinger biometric sensing device according to claim 10 wherein saidmemory is capable of storing error compensation data to account for tilterrors.
 12. The finger biometric sensing device according to claim 10further comprising a dielectric layer over said array of fingerbiometric sensing pixel electrodes; wherein said dielectric layer has anon-uniform thickness; and wherein said memory is capable of storingerror compensation data to account for the non-uniform thickness of saiddielectric layer.
 13. The finger biometric sensing device according toclaim 10 wherein said memory is capable of storing error compensationdata to account for fixed pattern noise errors.
 14. The finger biometricsensing device according to claim 10 wherein said memory is capable ofstoring error compensation data to account for a gain stage offseterror.
 15. The finger biometric sensing device according to claim 10wherein said at least one gain stage comprises a plurality thereofcoupled together in series and defining a summing node between a pair ofadjacent ones of said plurality of gain stages; and wherein said DAC iscoupled to said summing node.
 16. The finger biometric sensing deviceaccording to claim 10 further comprising switching circuitry coupled tosaid array of finger biometric sensing pixel electrodes and said atleast one gain stage capable of sequentially generating output data foradjacent regions of said array of finger biometric sensing pixelelectrodes.
 17. A method of error compensation in a finger biometricsensing device comprising an array of finger biometric sensing pixelelectrodes and at least one gain stage coupled to the array of fingerbiometric sensing pixel electrodes, the method comprising: using errorcompensation circuitry comprising a memory capable of storing errorcompensation data and a digital-to-analog converter (DAC) cooperatingwith the memory and coupled to the at least one gain stage to compensatefor at least one error based upon the stored error compensation data.18. The method according to claim 17 wherein the memory is capable ofstoring error compensation data to account for tilt errors.
 19. Themethod according to claim 17 further comprising a dielectric layer overthe array of finger biometric sensing pixel electrodes; wherein thedielectric layer has a non-uniform thickness; and wherein the memory iscapable of storing error compensation data to account for thenon-uniform thickness of the dielectric layer.
 20. The method accordingto claim 17 wherein the memory is capable of storing error compensationdata to account for fixed pattern noise errors.
 21. The method accordingto claim 17 wherein the memory is capable of storing error compensationdata to account for a gain stage offset error.
 22. The method accordingto claim 17 wherein the at least one gain stage comprises a pluralitythereof coupled together in series and defining a summing node between apair of adjacent ones of the plurality of gain stages; and wherein theDAC is coupled to the summing node.
 23. The method according to claim 17further comprising using control circuitry to adjust the at least onegain stage so that an output thereof is within a dynamic of an outputanalog-to-digital (ADC) coupled to the at least one gain stage andhaving a dynamic range.
 24. The method according to claim 17 furthercomprising using switching circuitry coupled to the array of fingerbiometric sensing pixel electrodes and the at least gain stage capableto sequentially generate output data for adjacent regions of the arrayof finger biometric sensing pixel electrodes.